Research so far has been inconclusive about the effect of losing and gaining productive peers on one’s own output. This column defines peers in three distinct ways and checks which types of peers matter, focusing on mathematicians shortly after the collapse of the Soviet Union. Losing intellectual competitors results in an increase in one’s output, whereas losing collaborators reduces it. Competition for resources and positive spillovers from high-quality peers are simultaneously at force, explaining the divergent findings in the peer effects literature.

Stars have direct impact on local economies. They can also indirectly affect growth in a positive way. This column examines the effect of academic star arrivals on the departmental knowledge productivity. Department-level output increases by 54% after the arrival of the star. The post-arrival quality of the joiners is also positively affected, displaying an increase of 68%. These star effects are largest at mid-ranked institutions.